The present invention relates to the field of thermal printing. More particularly, it addresses the technical task of eliminating certain artifacts appearing in thermal prints as a result of variations in the tension of a dye-bearing donor web during the printing operation.
In the thermal printing process, a dye-bearing donor web is brought into contact with a dye-receiving print media at a print zone. Thermal printing is effected by contacting the donor web with a multi-element print head which spans the donor web in a direction transverse to the direction of web travel. The print head typically comprises a linear array of closely spaced resistive elements, each being independently addressable by an applied current to heat that portion of the donor web directly opposite and thereby cause dye to transfer from the donor web to the print media. To maintain intimate contact between the donor web and print media during this printing operation, the donor web and print media are partially wrapped over the surface of a rotatably-driven platen roller, sometimes referred to as a "print drum". The print drum is commonly driven by a precision stepper motor so that the spacing between adjacent image lines can be precisely controlled. Most often, the take-up spool for the donor web is rotatably driven by a far less-expensive DC motor, since its function is simply to accumulate expended donor web. The donor web is supplied by a rotatably mounted supply spool, and a clutching arrangement is used to control the drag on the supply spool so as to prevent free-wheeling of the supply spool under the influence of the take-up spool motor.
In thermal printing apparatus of the above type, it has been observed that the print quality is influenced considerably by tension variations in the donor web during printing. When web-tension varies during printing, an artifact known as "banding" appears in the thermal print. Ideally, the pulling tension exerted on the donor web by the take-up spool should be maintained perfectly uniform throughout the printing cycle. Unfortunately, this ideal is very difficult to achieve, especially when relatively low-cost drive motors are used to effect take-up spool rotation. Also, the diameter of the take-up spool has a variable effect on web tension. As prints are made, the take-up spool diameter gradually increases, thereby altering the web tension.
In the commonly assigned U.S. patent application Ser. No. 504,445 entitled Thermal Printing Apparatus With Tensionless Donor Web During Printing, filed on Apr. 4, 1990 in the name of Stanley W. Stephenson, there is disclosed a thermal printer in which the tension in the donor web downstream of the print zone is reduced to zero during each printing operation. This tensionless condition virtually eliminates the banding artifact and is achieved by rotating the take-up spool at a rate slower than the rate at which the donor web is payed-out from the print zone by a rotatably driven print drum. A two-speed motor is used to rotate the take-up spool at two discrete rates, i.e., a first rate which is sufficiently slow as to produce, during each printing cycle, web slack between the print zone and the take-up spool, and a second rate which is sufficiently fast as to eliminate all web slack between printing cycles.
While the above-noted two-speed motor control apparatus of Stephenson functions well to provide the desired tensionless condition of a donor web in a thermal printer, it is not without limitations. As noted above, the donor web take-up spool gradually increases in diameter as more and more prints are made. The effect of this increase in take-up spool diameter is that the expended donor web is accumulated at an ever-increasing rate, even though the take-up spool rotates at a fixed angular velocity. To assure that a certain minimal slack is provided between the print zone and the take-up spool regardless of the take-up spool diameter, it is necessary to produce considerably more web slack when the take-up spool diameter is at a minimum than when it is at a maximum. Since certain physical constraints within the printer can limit the tolerable amount of web slack, it is necessary to either limit the diameter of the donor web supply and, hence, the maximum diameter of the take-up spool, thereby requiring more frequent interruptions in the printing operation to change the donor web supply and take-up, or to accept a certain amount of artifacts in the prints produced by the end portion of a relatively large donor web supply.